Probably the most popular type of patterns at or below the visible wave length of light are holograms. While other types exist such as diffraction gratings, it will be simpler to talk generally abut holograms since it is not the particular kind of this pattern of concern here nor the preparation of a particular pattern. Instead it is the process of replicating these patterns of this size from an original.
Holography is a fairly new technology. It was discovered about forty years ago and only in the past ten years have commercial applications appeared. Very simply described holography is the recording of a wave or wave front. It is a technical process for forming a visual record of an object wherein the image produced is three dimensional and appears as solid as the original object. There are several types of holograms but the main interest of the present invention is in the phase type embossed holograms of which one commonly known type is called a white light hologram.
An original holographic recording requires a certain amount of time to produce and thus the recording process does not lend itself readily to mass production. Consequently processes have been developed for converting original holographic recordings into master holograms from which replications can be made.
One known method of preparing a master phase embossed hologram involves the following steps:
1. Preparation of a hologram on photo resist emulsion film through an original interference pattern by laser exposure;
2. Developing the photo resist whereby the interference pattern is now a series of ultrafine ridges;
3. Electrodepositing metal on the interference pattern to build up a metal mold or master; and
4. Removing the master phase embossed hologram from the developed photo resist film.
The master can now be used to impress the interference pattern into plastic for large scale production of reflection holograms. This is commonly done by what is known as the hot stamp process. A thermoplastic deformable plastic film that has been first metallized is placed in contact with the pattern surface of the master hologram. The composite is pressed or stamped together while heat is used to soften the plastic so that it deforms sufficiently to the interference pattern. The two layers are then separated with the interference pattern in the plastic film. The metallic coating functions like a mirror and reflects white light waves through the interference pattern to create the changing images of the pattern.
Though the hot stamp process for replication is the process most commonly used, it suffers from several serious disadvantages. The first disadvantage relates to the materials involved. A requisite for the process is a deformable plastic film of the thermoplastic type. During the replicating process, the heat used must be carefully controlled. If not enough heat is used, the plastic film will not deform. If the heat is excessive, the image will be "flowed away." Even when replicating conditions are carefully controlled, the thermal stability of the resultant reproduction must be considered. This reproduction is still thermoplastic and can be again deformed if subjected to excessive heat. Non-thermoplastic materials which could provide many desirable properties to the finished product cannot be utilized in the hot stamping process.
Another disadvantage of the hot stamp process is that it is not a continuous operation. The hot stamp operation requires uniform heat and pressure in its operation as opposed to endless belts and casting drums.
In conventional hot stamp operations, a composite of the holographic master and the plastic film are placed in a press having at least one heated platen. The press is closed for a sufficient period of time to deform the plastic film. The press is opened and the composite is separated. The operation is limited to the size of the press. In turn, the size of the final hologram determines the number of hologram images that can be set in place on the hologram master. In contrast, endless belts and casting drums offer the opportunity for a continuous replication process. In using endless belts and drums, masters on the belts and drums are contacted with the plastic film that receive the image by means of a pressure roll which forms a nip with the belt or roll to join the two materials together.
Lastly, the hot stamp process is an embossing process that deforms the plastic coating. Usually an embossing process does not give 100% replication of patterns, especially the ultrafine patterns such as those with which the present invention is concerned.
U.S. Pat. Nos. 4,289,821 and 4,322,450 describe processes for the manufacture of release coated webs. In these process a web is coated with an electron beam curable coating which when cured has release properties. The freshly coated web is brought in contact with a continuous replicating surface and while in contact the web is irradiated on its back surface by an electron beam to cure the coating. Once the coating is cured it can be removed from the replicating surface and the pattern of the surface is replicated in the coating.
Representative of patterns made by the processes of these patents are leather and wood grain patterns as well as very uniform patterns which provide satin and low gloss finishes. Patterns such as these on the replicating surfaces and on the surfaces of replicated products are identifiable as peaks and valleys and they are measurable.
Using a Surtronic 3P profiling instrument, a set of 7 different patterns, representative of finer replications made by this process, was measured. The instrument was adapted with a data acquisition interface and related software to generate the average spacing between peaks and the average vertical distance between peaks and valleys. The instrument was adjusted to filter out any part of the patterns with a wave length of 250 microns or greater. For the set of samples, the peak to peak average was from 7.5 microns to 27.5 microns. The peak to valley average vertical distance was from 2.1 microns to 39.6 microns, far greater than the wave length of visible light.